Our goal is the selective destruction of tumor cells with potent cytotoxins conjugated to antibodies directed against tumor-specific antigens. We have found that one such toxin, diphtheria toxin, conjugated to tumor-specific antibodies, was capable of inducing permanent regressions of a hamster lymphoma. Current work is aimed at purifying diphtheria toxin conjugates to reduce their non-specific toxicity, and at developing conjugates prepared from other toxins. One such toxin, ricin, has exhibited selective cytotoxicity in vitro when conjugated to rabbit anti-HeLa antibodies and tested in cultures of HeLa cells. Attempts to study the antitumor effectiveness in vivo of antibody-ricin conjugates directed against tumor antigens are now being initiated.